1. Field of the Invention
The present invention relates to a method and apparatus for electro discharge machining. More particularly, the present invention relates to a method and apparatus for electro discharge micromachining used to machine a micro-diameter hole of a nozzle for discharging ink used for an ink-jet printer, a hole of a nozzle for producing a chemical fiber, a fuel injection nozzle used for an automobile engine, and the like.
2. Description of the Related Art
Recently, a hole of a nozzle for discharging ink used for an ink-jet printer, a nozzle for discharging material used for a chemical fiber production machine, a fuel injection nozzle used for an automobile engine, or the like, are becoming smaller and more precise. As such, there is a demand for hole machining capable of producing a number of nozzle holes having such a small diameter and precise geometry. Such machining typically requires several hours and high-precision positioning.
To achieve such high-precision hole machining, an electro discharge micromachining apparatus capable of hole micromachining using electro discharge may be employed. Such an electro discharge micromachining apparatus is provided with an electro discharge machining apparatus capable of controlling an electro discharge operation of the electro discharge micromachining apparatus with high precision so as to machine a microhole with electro discharge.
FIG. 8 shows a conventional electro discharge machining apparatus 1300. The electro discharge machining apparatus 1300 comprises: a holder 351 for holding a workpiece 350 to be subjected to electro discharge machining; an electrode 310 provided opposing the workpiece 350 held by the holder 351; a head 320 for holding the electrode 310; a capacitor 390 for supplying electric charge to the electrode 310 and the workpiece 350; a power source 360 for applying a voltage between opposite ends of the capacitor 390; electric resistances 370 and 380 provided between the capacitor 390 and the power source 360; a current detecting element 301 for detecting a current flowing between the workpiece 350 and the electrode 310; a control unit 300 for judging whether a short circuit occurs between the electrode 310 and the workpiece 350; a table 321 for supporting the head 320; a linear guide 330 capable of sliding in a Z axis direction for supporting the table 321; a ball screw 340 for sliding the table 321 in the Z axis direction due to axial rotation; a motor 315 for rotating the ball screw 340 about an axis of the ball screw 340; a motor driver 312 for driving the motor 315; and a motor controller 310 for instructing the motor driver 312 to drive the motor 315. The holder 351 may be a machining vessel supplied with a machining liquid.
The power source 360 which is a direct current power source applies a voltage between the opposite ends of the capacitor 390, so that electric charge is accumulated in the capacitor 390 in accordance with a charge time constant. The charge time constant is determined based on values of the electric resistances 370, 380 and the capacitor 390. The electric charge accumulated in the capacitor 390 is supplied to the electrode 310 and the workpiece 350, so that electro discharge occurs between the electrode 310 and the workpiece 350. Repetition of accumulation of electric charge to the capacitor 390 and supply of the accumulated electric charge to the electrode 310 and the workpiece 350 in accordance with the charge time constant leads to generation of pulse electro discharge between the electrode 310 and the workpiece 350. The workpiece 350 is machined by the pulse electro discharge.
Machining of the workpiece 350 is initiated by a machining initiation command by a user""s key input, for example. The machining initiation command leads to accumulation of electric charge into the capacitor 390, while the motor controller 310 outputs to the motor driver 312 a command to drive the motor 315 so as to move the electrode 310 downward. The motor drive causes the distance between the electrode 310 and the workpiece 350 to be small. When the distance becomes such that an electro discharge can be generated, pulse electro discharge is generated between the electrode 310 and the workpiece 350 to start machining of the workpiece 350.
In the electro discharge machining apparatus 1300, the current detecting element 301 detects a current flowing between the power source 360 and the capacitor 390. The current detecting element 301 may also detect a voltage between the electrode 310 and the workpiece 350. The current detecting element 301 outputs a result of detecting the current to the control unit 300.
The control unit 300 judges, based on the current detecting result, whether a short circuit occurs between the electrode 310 and the workpiece 350. If the control unit 300 judges that a short circuit occurs, the control unit 300 outputs a short circuit detection signal indicating the short circuit state to the motor controller 310. In accordance with the short circuit detection signal, the motor controller 310 outputs to the motor driver 312 a command to drive the motor 315 so as to elevate the workpiece 390 as long as a short circuit continues between the electrode 310 and the workpiece 350. The electrode 310 continues to be elevated in the Z axis direction until the short circuit halts between the workpiece 350 and the electrode 310.
When by the elevation of the electrode 310 in the Z axis direction, a mechanical isolation distance between the workpiece 350 and the electrode 310 is secured, the short circuit halts. When the control unit 300 detects the halt of the short circuit, the motor driver 312 drives the motor 315 to move the electrode 310 downward to a position for initiation of electro discharge, so that electro discharge machining is resumed and continued.
By the above-described electro discharge operation, electro discharge machining is carried out for the workpiece 350.
To achieve a high level of roundness in hole machining, the electrode 310 is rotated by a spindle motor during electro discharge machining. Typically, such machining is carried out for several hours.
However, in the above-described conventional electro discharge machining apparatus 1300, every time a short circuit occurs, the electrode 310 moves upward and downward so as to halt the short circuit. Such upward and downward operations require much time, leading to an increase in the time required for overall machining processes.
Further, in the electro discharge machining apparatus 1300, a high voltage remains applied between the workpiece 350 and the electrode 310 during a period of time from detection of the occurrence of a short circuit to restart of electro discharge machining. When a machining liquid (e.g., deionized water) supplied into the holder 351 (i.e., a machining vessel) is used to cool heat generated by electro discharge and remove swarfs during electro discharge machining, electrolysis occurs between the workpiece 350 and the electrode 310 due to a high voltage applied between the workpiece 350 and the electrode 310. Abnormal machining occurs due to the generation of electrolysis in addition to intended machining, leading to a reduction in quality of the machined workpiece 350.
Moreover, in the electro discharge machining apparatus 1300, the continuous rotation of the motor 315 and the spindle motor generates heat which in turn propagates through the head 320, the ball screw 340, or the linear guide 330 made of stainless steel, casting iron (FC material), or the like which, as a result, thermally expands. Such thermal expansion causes displacement of the electrode 310, leading to a reduction in machining precision. For example, it is assumed that the head 320 is made of stainless steel having the coefficient of thermal expansion of 17xc3x9710xe2x88x926 and has a length of about 50 mm. In this case, when the temperature of the head 320 is increased, for example, by three degrees due to heat generated by the spindle motor, the electrode 310 will be displaced from a machining position by 2.5 xcexcm.
The present invention is provided to solve the above-described conventional problems. An objective of the present invention is to instantaneously halt a short circuit between a workpiece and an electrode or to prevent occurrence of a short circuit between the workpiece and the electrode, thereby improving a machining rate. Another objective of the present invention is to, in electro discharge machining, use a machining liquid, such as for example deionized water, to prevent electrolysis of an electrode and a workpiece which would otherwise cause abnormal machining, thereby improving the machining quality of the workpiece. Still another objective of the present invention is to prevent displacement of an electrode due to heat generation of a motor during a long machining operation, thereby achieving high-precision positioning of the electrode.
According to one aspect of the present invention, an electro discharge machining apparatus comprises a first holder for holding a workpiece to be subjected to electro discharge machining, an electrode provided opposing the workpiece held by the first holder, a capacitor for supplying electric charge to the electrode and the workpiece so as to generate pulse electro discharge between the workpiece and the electrode, a power source for applying a voltage to the capacitor, a switching element provided between the capacitor, and at least one of the electrode, the workpiece and the power source, a switch controller for controlling on and off of the switching element, a current detecting element for detecting a current flowing between the workpiece and the electrode, and a control unit for, based on the current detected by the current detecting element, judging whether or not a short circuit occurs between the workpiece and the electrode. When it is judged that a short circuit occurs between the workpiece and the electrode, the switch controller switches off the switching element for a predetermined time.
In one embodiment of this invention, the electro discharge machining apparatus further comprises a drive control unit for moving a position of the electrode. When it is judged that a short circuit occurs between the workpiece and the electrode, the drive control unit moves the position of the electrode so that a distance between the workpiece and the electrode is increased.
In one embodiment of this invention, the electro discharge machining apparatus further comprises a motor for rotating the electrode, and a second holder for holding the motor and the electrode. The second holder is made of a low thermal expansive substance.
In one embodiment of this invention, the coefficient of thermal expansion of the low thermal expansive substance is 5xc3x9710xe2x88x926 or less.
In one embodiment of this invention, the low thermal expansive substance is an invar alloy.
In one embodiment of this invention, the electro discharge machining apparatus further comprises a motor for rotating the electrode, and a second holder for holding the motor and the electrode. An thermal isolator is equipped between the motor and the second holder.
In one embodiment of this invention, the electro discharge machining apparatus further comprises a motor for rotating the electrode, and a second holder for holding the electrode. Space is set between the motor and the second holder.
According to another aspect of the present invention, an electro discharge machining apparatus comprises a first holder for holding a workpiece to be subjected to electro discharge machining, an electrode provided opposing the workpiece held by the first holder, a capacitor for supplying electric charge to the electrode and the workpiece so as to generate pulse electro discharge between the workpiece and the electrode, a power source for applying a voltage to the capacitor, a switching element provided between the capacitor, and at least one of the electrode, the workpiece and the power source, and a switch controller for controlling on and off of the switching element. The switching element alternately is turned on and off so that duration of the pulse electro discharge is less than or equal to a predetermined time.
In one embodiment of this invention, the predetermined time is a time from the beginning of the pulse electro discharge to the onset of electrolysis in one of the electrode and the workpiece.
In one embodiment of this invention, the electro discharge machining apparatus further comprises a motor for rotating the electrode, and a second holder for holding the motor and the electrode. The second holder is made of a low thermal expansive substance.
In one embodiment of this invention, the coefficient of thermal expansion of the low thermal expansive substance is 5xc3x9710xe2x88x926 or less.
In one embodiment of this invention, the low thermal expansive substance is an invar alloy.
In one embodiment of this invention, the electro discharge machining apparatus further comprises a motor for rotating the electrode, and a second holder for holding the motor and the electrode. An thermal isolator is equipped between the motor and the second holder.
In one embodiment of this invention, the electro discharge machining apparatus further comprises a motor for rotating the electrode, and a second holder for holding the electrode. Space is set between the motor and the second holder.
According to another aspect of the present invention, an electro discharge machining apparatus comprises a first holder for holding a workpiece to be subjected to electro discharge machining, an electrode provided opposing the workpiece held by the first holder, a capacitor for supplying electric charge to the electrode and the workpiece so as to generate pulse electro discharge between the workpiece and the electrode, a power source for applying a voltage to the capacitor, a current detecting element for detecting a current flowing between the workpiece and the electrode, a control unit for, based on the current detected by the current detecting element, judging whether or not a period of the pulse electro discharge is less than or equal to a predetermined period, and an adjuster for, when it is judged that the period of the pulse electro discharge is less than or equal to the predetermined period, adjusting an electric resistance value between the power source and the capacitor so that the period of the pulse electro discharge is greater than the predetermined period.
In one embodiment of this invention, the electro discharge machining apparatus further comprises a motor for rotating the electrode, and a second holder for holding the motor and the electrode. The second holder is made of a low thermal expansive substance.
In one embodiment of this invention, the coefficient of thermal expansion of the low thermal expansive substance is 5xc3x97106 or less.
In one embodiment of this invention, the low thermal expansive substance is an invar alloy.
In one embodiment of this invention, the electro discharge machining apparatus further comprises a motor for rotating the electrode, and a second holder for holding the motor and the electrode. An thermal isolator is equipped between the motor and the second holder.
In one embodiment of this invention, the electro discharge machining apparatus further comprises a motor for rotating the electrode, and a second holder for holding the electrode. Space is set between the motor and the second holder.
According to another aspect of the present invention, an electro discharge machining apparatus comprises a first holder for holding a workpiece to be subjected to electro discharge machining, an electrode provided opposing the workpiece held by the first holder, a capacitor for supplying electric charge to the electrode and the workpiece so as to generate pulse electro discharge between the workpiece and the electrode, a power source for applying a voltage to the capacitor, a current detecting element for detecting a current flowing between the workpiece and the electrode, a control unit for, based on the current detected by the current detecting element, judging whether or not a short circuit has occurred for a predetermined time between the workpiece and the electrode, and an adjuster for, when it is judged that a short circuit has occurred for the predetermined time between the workpiece and the electrode, adjusting an electric resistance value between the power source and the capacitor so that a period of the pulse electro discharge is elongated.
In one embodiment of this invention, the electro discharge machining apparatus further comprises a motor for rotating the electrode, and a second holder for holding the motor and the electrode. The second holder is made of a low thermal expansive substance.
In one embodiment of this invention, the coefficient of thermal expansion of the low thermal expansive substance is 5xc3x9710xe2x88x926 or less.
In one embodiment of this invention, the low thermal expansive substance is an invar alloy.
In one embodiment of this invention, the electro discharge machining apparatus further comprises a motor for rotating the electrode, and a second holder for holding the motor and the electrode. An thermal isolator is equipped between the motor and the second holder.
In one embodiment of this invention, the electro discharge machining apparatus further comprises a motor for rotating the electrode, and a second holder for holding the electrode. Space is set between the motor and the second holder.